Periodical cicadas, with voices that are capable of drowning out lawn mowers and making normal conversation impossible, are back. But their noise and the tree damage they cause won't be as severe as it was four years ago.Cicadas come in two varieties, annual and periodical. Every year, in late summer, some of the annual or "dog-day" cicadas living in a given area emerge from the ground, where they have lived as larvae for years, sucking juice from plant roots to nourish themselves. When they emerge, they molt and unfold their wings. Males fly into nearby trees, where they sing to attract mates. Their rhythmic, monotonous song is a pleasant reminder that cooler weather is not far off.Periodical cicadas are of two types. The life cycle of one takes 13 years to complete, the other 17 years. Their synchronized mating behavior results in the formation of regional "broods," or communities of the same periodical cicadas that inhabit a well--defined geographical area and emerge in the same year. Insect scientists--entomologists--have identified 15 broods nationwide and mapped their ranges.This year, some of the 13--year periodical cicadas emerged in an irregular region reaching from central Illinois to the Gulf of Mexico and from central Kentucky to Missouri. In Missouri, the Missouri Department of Conservation has received reports of periodical cicada sightings in Bollinger, Dent, Laclede, Oregon and Ripley counties, as well as Boone and Cole counties in north central Missouri. Smaller numbers of the insects likely emerged in an area extending to extreme southwestern Missouri.In 1998, Missouri witnessed a historic simultaneous emergence of two cicada broods, both the 13--year cicadas and 17--year type. The simultaneous convergence of two broods occurs only once every 211 years, but when it does the results are spectacular. Millions of red--eyed cicada nymphs crawl out of their burrows, transform into adults and begin singing, mating and laying eggs.Besides driving some people bonkers, periodical cicadas also can be hard on trees. Females use a knife--like appendage on their abdomens to slice into pencil--sized tree branches where they deposit their eggs. Damage caused by the egg laying kills many branch tips. The dead tips droop and turn brown, a phenomenon known as "flagging."In the coming weeks, residents of southeastern Missouri may see flagging in affected trees. Conservation Department Forest Entomologist Rob Lawrence says giving damaged trees proper care can help them heal with little harm. "As far as the health of the tree, the flagging is not a major concern," says Lawrence. "Large trees should recover easily. Young trees or trees that are in poor health due to other insects, disease or injury will require more care. It's best just to keep them well watered and place mulch around them to retain moisture and help avoid damage from lawnmowers and trimmers."Water is essential because it helps the trees deal with stress due to damaged branches and leaves. The entomologist advises homeowners to saturate soil near the drip line of damaged trees regularly during dry periods.Mulch should be placed at least three inches away from the base of the tree. Lawrence says placing mulch against the bark could promote the growth of fungal or bacterial diseases that could further damage a tree.Even with proper care, some of the cicada--damaged branches will continue to turn brown as late as next spring or summer, but it is unlikely that the entire tree will succumb.The leading theory to explain periodical cicadas' synchronized emergence is that coming out only once every 13 or 17 years prevents birds and other predators from fully exploiting the food source the insects' juicy little bodies represent. Their enemies can only eat so many cicadas before the insects finish mating, and without the superabundance of cicada snacks to sustain them year after year, predator's numbers remain too low to put much of a dent in periodic emergencies.If you think eluding predators by infrequent emergence is impressive, consider another fact. Both 13 and 17 are prime numbers -- they can be divided only by the number one and themselves. This means that a predator species -- say a wasp, that staggers its reproduction by a few years in "pursuit" of cicada prey, can't just luck into a free lunch by developing a reproductive cycle whose multiple happens to coincide with that of their prey.Even with several broods in Missouri, concurrent emergencies are rare. Prior to 1988, the last time it happened was 1946.The 1998 convergence was particularly unusual because it involved two very large broods. Their combined distributions covered the entire state. It was the first time since 1777 that these two groups emerged at once. Less spectacular cicada emergencies are expected in central Missouri in 2006 (17-year cycle), in eastern Missouri in 2011, in northern Missouri in 2014 and in west--central and southeast Missouri in 2015.